Electrical service providers such as electrical utilities employ electricity meters to monitor energy consumption by customers (or other entities). Electricity meters track the amount of energy consumed by a load (e.g. the customer), typically measured in kilowatt-hours (“kwh”), at each customer's facility. The service provider uses the consumption information primarily for billing, but also for resource allocation forecasting and other purposes.
Electricity meter tampering is a form of energy theft that can have significant impact on costs for utilities and paying customers. Electricity meter tampering typically involves modification of the meter to bypass, alter or disconnect the consumption metering function of the meter, such that less than all of the actual energy consumption is registered.
An emerging type of meter tampering involves the use of a high power magnet. In particular, electricity meters using standard current transformers (“CTs”) are susceptible to tampering using high-power magnetics. To this end, the CT current sensor behavior can be impacted if a large magnet such as a neodymium iron boron magnet (sometimes referred to as a rare earth magnet) is placed in close proximity. The high intensity magnetic field will result in an error in sensing current and potentially a significant under-registration of (and consequent underbilling for) energy consumed.
To combat this issue, it has been known to place a ferromagnetic shielding structure around the CT or the entire interior of the meter to reduce the impact of the magnetic field on interior components. However, magnetic shielding requires additional material, a careful design to meet high voltage insulation requirements and labor cost, and undesirably increases the cost, manufacturability complexity and weight of the meter.
It is also known to employ a magnetic sensor to detect and flag a potential tampering situation. See U.S. Patent Publication No. 2010/0072990 and U.S. Pat. No. 7,495,555 for examples of this method. In at least some prior art devices, a Hall Effect sensor is used to detect placement of a magnet that could be due to tampering.
One drawback of many prior art meters that employ magnetic sensors is the need to distinguish magnetic signals associated with legitimate activity from those associated with intentional or unintentional tampering. For example, it is known to use a permanent magnet switch to provide rudimentary input to a meter in order to obtain displays of different values. It is also known to use magnets to hold external communication probes to a meter optical port. Both uses are legitimate, and typically do not alter the energy measurement in an appreciable manner.
One method of distinguishing legitimate uses of magnetic fields from illicit uses of magnetic fields is to set a threshold for detected magnetic fields that exceeds those normally encountered in legitimate uses. Thus, very high magnetic fields often indicative of tampering will cause the meter to record, display and/or communicate a potential tamper situation, while lower magnetic fields often associated with communication probes and magnetic switch inputs are ignored. A drawback to this approach is that at least some true tampering situations will not cause a detected magnetic field that exceeds the threshold. These tampering situations can result in energy metering inaccuracy that can persist for long periods of time.
There is a need, therefore, for a more cost-effective approach to combating meter tampering that further reduces the revenue lost due to tampering or other magnetic field-induced inaccuracy.